I have used Thrifty a couple of times and was perfectly satisfied with both the service and the cars.
The location is off-airport, but is one of the closest off-airport rental agencies; the shuttle bus ride was only about 5 minutes. Returning the car, the place was easy to find, and the procedure for returning was easy and quick.
Debden is absolutely correct; sign up for the free Blue Chip club, which allows you to fill out a master rental agreement ahead of time so you won't have to when you pick up the car - and also allows you to skip the insurace sales pitch.
If you are a member of Sam's Club, pick up their travel magazine for Thrifty discount codes and coupons which will save you a lot of money. Using the Sam's codes and coupons I have often gotten the best rates from Thrifty.